1. Field of the Invention
The present invention relates generally to the production of foamed plastic material and more particularly to the production of sheets, webs or strands with integral unmodified smooth skin from microcellular foamed plastic material, by which is meant a plastic material having uniformly distributed voids or cells of very small size, i.e. on the order of 2 to 25 microns.
2. Description of the Prior Art
As disclosed in U.S. Pat. No. 4,473,665, issued on Sept. 25, 1984, and assigned to the Massachusetts Institute of Technology, foamed plastic material with very small and uniformly distributed voids or cells can be produced by impregnating a plastic material under pressure with an inert gas, which nucleates and expands to provide the desired cellular structure when the material is at a temperature within the range of its glass transition temperature and the pressure is reduced. The resulting product is referred to as microcellular foam, which characterizes a product having uniformly distributed voids or closed cells of very small size, e.g. 2 to 25 microns. In one embodiment, a batch process is disclosed in which a previously formed plastic sheet or other article is impregnated with gas under pressure, the pressure is reduced to ambient, the material is heated to a softening point to effect foaming, and the material is quenched to terminate foaming when the desired degree of foaming has been achieved. As soon as depressurization occurs, the absorbed gas begins to diffuse out of the impregnated plastic. Accordingly, for maximum foaming, the heating of the material to its foaming temperature should take place as soon as possible after depressurization. However, because the gas diffuses most rapidly from the surface regions of the material, an appropriate delay between depressurization and reheating of the material will result in a cellular core with an integral, unmodified laminar skin. Because the skin results from the inherent inability of this surface portion of the material to foam, the skin surface is defined by the extruder, and i.e. thus as smooth as the surface of a corresponding extruded unfoamed sheet. Furthermore, during the subsequent reheating of the sheet to induce foaming, the entire sheet can be heated to its softening or glass transition temperature, whereby the sheet surfaces are free to expand in response to the foaming, i.e., the entire sheet is free to expand in all directions without inducing significant stresses in the sheet to the above-described process.
In contrast, there are basically two other previously known techniques for forming a skin on foamed plastic material. In one of these techniques, the foam surface is reconstituted by using heat and pressure to soften and recompress the surface material, which can be done in the foaming die itself or by a secondary operation. In the other technique, either before foaming, prior to or during the foaming stage, the material is brought into contact with a relatively cool surface, typically within the foaming die; so that the surface of the material is cooled below its foaming temperature before it has foamed and the resulting skin is then stretched by the expanding foam to accommodate the final size of the articles. While both of these techniques are commonly used to provide foamed articles with a very thin cosmetic skin, such a surface is generally not entirely smooth. Furthermore, if either of these techniques is employed to produce a substantially thicker skin, the cycle or processing time can become prohibitively long and the structural integrity of the material is likely to be impaired because of the unequal expansion, contraction and pressures experienced between the skin regions and the cellular core. Additionally, in using either of these two basic techniques, it is very difficult to selectively control the thickness of the desired skin without altering some other parameter that should preferably be determined by other considerations, e.g. extrusion speed or extrusion temperature. Such control is particularly important in the case of relatively thin webs e.g. 10-25 mils which are made of a transparent material in which case it may be critical that the foamed core be sufficiently thick to provide uniform opacity but also that the integral laminar skin be as thick as possible to provide desired physical characteristics.
To distinguish the material produced in accordance with the present invention, which employs the degassing process disclosed in the M.I.T. patent from that produced by the two techniques just described, the term "integral unmodified laminar skin" is intended to characterize the skin as being formed from the same parent material as the cellular core, as opposed to being laminated to the core (integral); to characterize the skin as not having been foamed and then reconstituted as not having been foamed and then stretched significantly at below its softening temperature (unmodified); and, to characterize the skin as having sufficient thickness to comprise a definite lamina, e.g. at least 1 mil (laminar).
While the batch technique of the M.I.T. patent will produce the desired type of skin in an article with a microcellular foam core, it is obviously suitable only for experimental or very limited production purposes and cannot possible be economically viable for the production of commercial quantities of foamed plastic sheet or the like.
An alternative process, disclosed in the same M.I.T. patent discussed above, involves extruding a web of pre-impregnated molten plastic material into a pressurized chamber, in which it passes first through a heated bath that prevents so-called "freezing-off", i.e. adhesion to the extruder die and, then, through a cooling bath, that cools it below its foaming or glass transition temperature. Thereupon, it passes through a pressure seal and into a reheating bath at ambient pressure, which reheats it to an appropriate temperature to induce foaming. The plastic can be impregnated either by previously exposing the plastic pellets to pressurized gas before they are introduced into the extruder or by injecting gas into the molten plastic within the extruder. It should be noted that the term "impregnated" as used herein does not necessarily mean that the cooled material has been completely saturated to the solubility limit when it is initially depressurized, but rather, that a substantial portion of that amount of gas is absorbed or dissolved generally homogeneously throughout the material. Although very desirable from a continuous production standpoint, this technique also suffers from several disadvantages, e.g. the difficulty in threading the initial extruded web along a convoluted pathway through inaccessible pressurized portions of the apparatus, difficulties in controlling tension within the apparatus, and problems associated with sealing the web passageway to prevent leakage of pressurization. More significant, however, is the fact that because the reheating of the web to produce foaming occurs immediately adjacent the exit seal, it is not possible to vary the diffusion of gas out of the material prior to foaming. Accordingly, the finished material has little or no unmodified laminar skin and there is no way in which skin thickness can be controlled except to a limited extent, by varying the extrusion speed and for the temperature of the foaming medium, either or both of which may be undesirable for other reasons.
A somewhat analogous process for producing foamed extruded plastic material is disclosed in Japanese Patent Kokais Nos. SHO 59(1984)-169824, published on Sept. 25, 1984 and SHO 60(1985)-99629, published on June 3, 1985, both of which are assigned to Mitsubishi Yuka Co., Ltd., Tokyo, Japan. In accordance with these disclosures, a molten resin (e.g. polystyrene) is blended or impregnated with a volatile foaming or blowing agent and is extruded at elevated pressure into a long die. In the portion of the die nearest the extruder, the material is maintained under sufficiently high pressure to prevent foaming as the material is cooled to its optimum foaming temperature while passing along the die. To counteract the frictional resistance caused by the corresponding increase in viscosity of the material, a lubricant is injected between the die surfaces and the adjacent faces of the plastic material. A restriction is preferably employed at the end of the first portion of the die to aid in maintaining the pressure in that die region and, beyond the restriction, the die throat is expanded to define the size of the desired finished product. As the plastic enters the larger portion of the die, it foams and is further cooled to a dimensionally stable temperature before exiting the die, with its passage through the die still being assisted by the previously mentioned lubricant.
Another closely related process, for providing foamed plastic insulation on electrical wire, is disclosed in U.S. Pat. No. 3,988,404, issued Oct. 26, 1976 and assigned to the Furukawa Electric Co., Ltd., Tokyo, Japan. According to this disclosure, plastic pellets are pressurized with a gas in a two stage pressurization process and, while maintained under pressure, the pellets are fed to an extruder through which the electrical wire is drawn. As the material, cooled to its desired foaming temperature, is extruded out of the die around the wire, it foams to provide an insulating layer that is described as having minute and homogenous cells.
In investigating the techniques disclosed in the foregoing patents assigned to Mitsubishi Yuka Co. and the Furukawa Electric Co., Ltd., we have found that smaller, more symmetrical and more uniformly distributed cells appear to be produced by reheating a rod or web of plastic material that was impregnated with gas and cooled to below its foaming temperature before being allowed to foam than by allowing the material, at a foaming temperature, to expand into an enlarged die region or into the atmosphere. More particularly, our observation has been that these latter prior art techniques tend to produce relatively elongate cells that are larger and less uniformly distributed; which adversely affects the physical properties of the finished product. Although other factors may be very significant, it seems likely that this difference in cell size, shape and distribution may be attributable principally to two distinctions between the prior art technique, and the technique to which the present invention is directed, namely that in the latter case the cells are nucleated and grown at a relatively low temperature, which produces smaller cells, and, that the reheating technique allows the material greater freedom to expand uniformly in all directions and thereby avoids internal stresses. It should also be noted that, any skin produced by these processes results primarily from either reconstituting the foam surface or from preventing foaming by chilling the surface of the material before the surface regions have foamed. In other words, such processes are not capable of producing integral unmodified laminar skin.